1. Field of the Invention
The present invention relates to a method of producing a display device, a display device, a method of producing a thin-film transistor substrate, and a thin-film transistor substrate. In particular, the present invention relates to a method of producing an active matrix-driving display device including organic electroluminescent elements, the structure thereof, a method of producing a thin-film transistor substrate that is suitably used in the display device, and a thin-film transistor substrate.
2. Description of the Related Art
In an active matrix-driving display device in which organic electroluminescent elements and pixel circuits connected thereto are arranged on a substrate, the luminance of each of the organic electroluminescent elements is determined by the amount of current of a thin-film transistor (TFT) constituting the pixel circuit. Therefore, in order to obtain a display device in which uneven luminance is suppressed and which has satisfactory display properties, it is important that variation in properties of the thin-film transistor be suppressed.
In the case where a channel region of a thin-film transistor is formed of polycrystalline silicon, the size of crystal grains present in the channel region is nonuniform, and thus transistor characteristics readily vary. Consequently, as a method of microcrystallizing a semiconductor thin film constituting the channel region to the extent that the size of the crystal grains is not nonuniform, crystallization annealing, in which an amorphous thin film is microcrystallized using a solid-state laser, has been performed.
FIG. 23 shows an arrangement on a thin-film transistor substrate for illustrating a step of the above-mentioned crystallization annealing. FIG. 24 shows the arrangement of two display pixels in the thin-film transistor substrate. As shown in these figures, in a process of producing a medium or small size display panel, for example, two display panels 2 are arranged on a single glass substrate 1. In this case, a short side of each of the display panels 2 is arranged so as to be parallel to a long side of the glass substrate 1. In each of the display panels 2, a display area 2a having a shape that is substantially similar to the shape of the display panel 2 is defined. In each of the display areas 2a, sub-pixels a each having a rectangular shape in plan view are arranged. The sub-pixels a are arranged such that a long side of each of the sub-pixels a is parallel to a short-side direction y of the display area 2a. Furthermore, these sub-pixels a constitute substantially square display pixels A each composed of a set of three sub-pixels a of red (R), green (G), and blue (B), the three sub-pixels a being arranged in a direction of a short side of each of the sub-pixels a.
In the display area 2a in each of the display panels 2, scanning lines 11 and power lines 12 are arranged in a long-side direction x of the display area 2a, and signal lines 13 are arranged so as to be perpendicular to the scanning lines 11 and the power lines 12. The sub-pixels a are arranged so as to correspond to intersecting portions of these lines. In each of the sub-pixels a, thin-film transistors Tr1′ and Tr2′ each including a gate electrode 14 extending parallel to the signal lines 13, and a capacitive element Cs are arranged. In this case, by reversely disposing an arrangement of adjacent sub-pixels a, a part of the power line 12 can be shared, and thus a distance between wirings in a display pixel can be increased. As a result, short-circuit caused by, for example, a mixing of dust in a production process and generation of defects of the display pixel can be suppressed to improve the yield. In particular, a pixel circuit for diving an organic electroluminescent element includes a large number of elements, and thus it is important to increase the distance between wirings.
In order to make the above-described thin-film transistor substrate, the step of crystallization annealing is performed as follows. First, as shown in FIG. 25, gate electrodes 14 composed of a first metal pattern 21 are formed in each of display areas 2a on a glass substrate 1, and other wiring portions composed of the first metal pattern 21, such as parts of signal lines 13 and lower electrodes of capacitive elements Cs, are also formed at the same time. Next, a gate insulating film 31 and an amorphous semiconductor thin film 32 are formed in that order so as to cover the first metal pattern 21. Furthermore, a buffer layer and a photothermal conversion layer (not shown) are optionally formed thereon. Next, the semiconductor thin film 32 is irradiated with a laser beam through these layers while scanning the laser beam. Thereby, a semiconductor thin film 32 portion corresponding to a portion irradiated with the laser beam is microcrystallized to form a semiconductor thin film 32A.
In this case, a scanning direction v (−v) of the laser beam is parallel to the long-side direction x of the display area 2a. That is, as shown in FIG. 23, the scanning direction v (−v) of the laser beam is parallel to a short side of the substrate 1. Accordingly, variation in energy of a laser beam Lh due to a long scanning distance of the laser beam can be prevented, thereby forming more uniform crystals. In this case, in the example shown in FIG. 24, a channel length direction of each of the thin-film transistors Tr1′ and Tr2′ corresponds to the scanning direction v (−v) of the laser beam.
After the above-described step of crystallization annealing, as shown in the plan view of FIG. 26A and the cross-sectional view of FIG. 26B, the semiconductor thin film 32A, which has been microcrystallized, is patterned so as to have a shape covering the gate electrode 14. Furthermore, an etching stopper layer 33 is formed so as to overlap with the gate electrode 14. Note that FIG. 26B is a cross-sectional view taken along line XXVIB-XXVIB in FIG. 26A. Next, a source/drain 34sd (shown in only the cross-sectional view) composed of an n-type semiconductor thin film is formed so that the source/drain 34sd overlaps with the semiconductor thin film 32A and is separately disposed on the etching stopper layer 33. Subsequently, a source electrode/drain electrode 22sd composed of a second metal pattern 22 is further formed, thus obtaining the thin-film transistors Tr1′ and Tr2′. Other wiring portions composed of the second metal pattern 22, for example, the scanning lines 11, the power lines 12, and upper electrodes of the capacitive elements Cs, all of which are shown in FIG. 24, are also formed together with the source electrode/drain electrode 22sd. 
In the above-mentioned crystallization annealing using a solid-state laser, a thermal diffusion length when a heat quantity necessary for crystallizing a semiconductor thin film is supplied is longer than that in the case of crystallization annealing using an excimer laser. Accordingly, an effect of heat conduction by the gate electrode 14, which is provided under the semiconductor thin film 32, is significant, and thus crystallinity of the semiconductor thin film 32 is affected.
Consequently, the following structure has been proposed. As shown in FIG. 26, the gate electrode 14 of the thin-film transistors Tr1′ and Tr2′ is provided so as to extend in the channel length direction (i.e., the dimension of the gate electrode 14 is increased in a width direction), and the extending portions ΔLs and ΔLd are used as heat transfer components. According to this structure, in the step of crystallization annealing described above, the effect of heat conduction by the gate electrode 14 is made to be uniform in a channel region overlapping with the gate electrode 14. Consequently, uniform crystallinity of the channel region can be realized (see Japanese Unexamined Patent Application Publication No. 2007-35964).